Optical security devices generally include at least two components: an arrangement of image icon elements and an arrangement of focusing elements. These components are disposed relative to each other such that they cooperate to produce a synthetic image when the arrangement of image icon elements or portions thereof are viewed through the arrangement of focusing elements or portions thereof. As used herein, all references to arrangements of image icon elements or focusing elements shall be understood to include reference to all of the elements in the arrangement or portions thereof, unless otherwise specified. Such devices may include an optical spacer disposed between the arrangement of image icon elements and the arrangement of focusing elements.
Such optical security devices enjoy high efficiency as anti-counterfeit technologies due largely in part to their diversity of visual effects, the complexity of the construction of such devices, the expense of the equipment, tooling and processes employed in their manufacture and the dynamic visual effects they produce. For example, optical security devices such as those described above often take a micro-structure approach to manufacture, where the arrangement of image icon elements is produced by forming micro-structured icon elements in a radiation cured liquid polymer material. These icon elements often take the form of microscopic voids or recesses formed in a polymeric material. Correspondingly, the arrangement of focusing elements is produced by forming microscopic lenses (hereinafter “lenses” or “micro-lenses”) in a radiation cured polymer. This often involves the arrangement of image icon elements and the arrangement of focusing elements being disposed on opposing sides of an optical spacer. The voids are then filled and/or coated with a contrasting material that provides color, reflective, refractive, diffractive or textured contrast (i.e., pigmented or metallic and/or reflective materials) for the voids, or portions thereof, compared to surrounding or neighboring regions in the arrangement of image icon elements.
Examples of optical security devices formed from a micro-structured approach are provided in U.S. Pat. Nos. 7,333,268 and 7,468,842. While these patents describe a micro-structured approach to forming the icon elements, where the icon elements are filled and/or coated, the filling/coating material is cured or otherwise solidified by non-directionally curing the filled/coated icon elements. The means of curing in these references is either applied directly (i.e., not through an optical spacer, arrangement of focusing elements, or other structural components of the optical security device) to the arrangement of image icon elements or is applied without the use of any directionally focused light such as collimated light. Such structures allow for large color blocks to be formed. However, while the synthetic images of such devices are viewable over a wide range of angles, color changes are more difficult to achieve and control.
To facilitate the cooperation of the arrangement of focusing elements with the arrangement of icon elements, these components are precisely aligned such that the viewing of the arrangement of image icon elements through the arrangement of focusing elements will produce a synthetic image. Such requirements for precise alignment of the arrangement of image icon element and the arrangement of focusing elements both relative to each other and across any optical spacer which may be present, adds to the complexity of the process of manufacture but limits the diversity of dynamic effects that can be produced. For example, such stringent alignment limitations often prevent the use of multiple contrasting materials such that an arrangement of micro-structured icon elements can produce synthetic images that vary in color.
Attempts have been made to improve the efficiency of these optical security devices; particularly as it relates to the use of micro-structured image icon elements to generate colorshifting images. For example, International Patent Application No. PCT/US2015/022907 provides what are described therein as flicker-like optical effects. Here, the micro-structured image icon elements are formed by forming voids in the polymeric material, applying a contrasting material to the voids, and then directionally curing these image icon elements; more specifically, curing the contrasting materials that are part of the image icon elements. To cure these image icon elements, collimated light is directed through an arrangement of focusing elements toward the arrangement of icon elements such that the collimated light impinges on the contrasting material thereby producing a pattern of cured contrasting material. This pattern is determined by the arrangement pattern of the arrangement of focusing elements and the cure angle (e.g., the angle at which collimated light impacts the lenses). Here, the arrangement of focusing elements is disposed relative to the arrangement of image icon elements such that when the collimated light is passed through the individual focusing elements at a desired angle, a pattern of cured contrasting material is generated in the arrangement of image icon elements. This pattern of cured contrasting material correlates to the arrangement pattern of the focusing elements in the arrangement of focusing elements such that the synthetic image produced by the pattern of cured contrasting material is observable only when viewed, at the cure angle, through the focusing elements. Another pattern of cured contrasting material may also be generated by washing out the uncured contrasting material to leave an uncured area in the voids, which are then filled with another contrasting material (i.e., different color, different material, or different texture). This other contrasting material can then be directionally cured by directing the collimated light, at a second and different cure angle, through the same arrangement of focusing elements thereby creating another pattern of cured contrasting material that correlates to the arrangement pattern of the arrangement of focusing elements. Again, the synthetic image produced by the other pattern of cured contrasting material is observable only when the arrangement of icon elements are viewed through the focusing elements at the second cure angle. While this allows for alignment of the arrangement of focusing elements with the arrangement of cured image icon elements and allows for the color of the synthetic images to change as the point of view changes, there remain some significant deficiencies.
For example, such an optical security device is limited in that the synthetic image generated can only be observed at the particular cure angles. Accordingly, the transition (e.g., color shift) from one contrasting material to another is not a smooth transition as an observer will notice the “snap” effect, for example, when a synthetic image is changed from one color to another. This snap effect leaves the security device susceptible to counterfeiters. Moreover, this single color limitation makes such devices more susceptible to counterfeiters.